Kaluza-Klein theory is an attempt to exhibit a transformation that charged particles travel on "straight" lines in a 5-dimensional space. The fifth dimension is supposed to be a very small circle. There are some experimental implications, but the circle is so small that we can't detect them (it's similar with string theories).

String theory tries to do the same thing with other forces; so it's not clear whether there's such a transformation. Forces are technically simpler than embedding extra dimensions that there are no evidence for.